Add short artifact handle (A1) so the model can edit by reference

Give each produced artifact a short virtual handle - A1, A2, ... - the n-th
artifact in the conversation or workflow run (case-insensitive, not stored). The
tools return it in their results, and the edit, rewrite, and input parameters
accept either the handle or the full id. A handle resolves only within the
caller's own conversation or run, and the resolved id is still checked against
that parent before any read, so it cannot reach another tenant's artifact. This
fixes the model creating a duplicate instead of a new version when asked to
edit an artifact.

tests/integration/test_workflow_code_node_e2e.py

@@ -153,11 +153,13 @@ def test_code_node_persists_artifact_reference_in_state(wired):
     list(engine._execute_code_node(node))
 
     ref = engine.state["report"]
-    # The state holds an artifact REFERENCE (JSON primitives), never bytes.
-    assert set(ref) == {"artifact_id", "version", "filename", "mime_type", "size"}
+    # The state holds an artifact REFERENCE (JSON primitives), never bytes. The
+    # short handle ``ref`` (A1) lets a later node address the artifact by position.
+    assert set(ref) == {"artifact_id", "version", "filename", "mime_type", "size", "ref"}
     assert ref["filename"] == "report.txt"
     assert ref["mime_type"] == "text/plain"
     assert ref["size"] == len(b"compliance ok")
+    assert ref["ref"] == "A1"
     assert all(not isinstance(v, (bytes, bytearray)) for v in ref.values())
     assert engine.state["node_code_1_output"] == ref